College Board-Aligned Original Notes
AP Chemistry 7.2: Calculating the equilibrium constant
Write and evaluate K expressions from equilibrium concentrations.
Aligned to Equilibrium from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.
What To Know
- K compares product and reactant concentrations at equilibrium.
- Pure solids and liquids are omitted from K expressions.
- The size of K indicates product-favored or reactant-favored equilibrium.
Detailed Notes
Calculating the equilibrium constant is part of Unit 7: Equilibrium. The main skill is to write and evaluate K expressions from equilibrium concentrations. Before answering, decide whether the prompt is asking for a particulate explanation, a mathematical setup, a graph interpretation, or a connection between more than one representation.
The first idea to keep straight is that k compares product and reactant concentrations at equilibrium. In the same topic, remember that pure solids and liquids are omitted from K expressions. A complete AP answer also uses the fact that the size of K indicates product-favored or reactant-favored equilibrium. These ideas should be tied to specific particles, charges, attractions, energy changes, or measured quantities rather than stated as isolated facts.
For calculations or symbolic work, anchor the solution with for aA + bB <-> cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b. Define what each quantity represents, substitute values with units, and check whether the sign, magnitude, charge balance, atom balance, or equilibrium direction makes chemical sense for this topic.
Use equilibrium concentrations, not initial concentrations, when calculating K. In a free-response explanation, state the chemistry concept first, show the relevant equation or representation, and then explain how the evidence supports the conclusion for calculating the equilibrium constant.
Key Vocabulary
Equilibrium constant
A value that describes the ratio of product and reactant activities at equilibrium.
Equilibrium expression
The mathematical form used to calculate K from equilibrium amounts.
Product-favored
An equilibrium position with relatively more products than reactants.
Reactant-favored
An equilibrium position with relatively more reactants than products.
Useful Relationships
Worked Study Approach
Use equilibrium concentrations, not initial concentrations, when calculating K.
Common Mistakes
- Using a memorized rule without explaining the chemical reason behind it.
- Forgetting to conserve atoms, charge, energy, or units when the topic involves calculations.
- Mixing up particle-level explanations with macroscopic observations.
Quick Practice
How would you explain Calculating the equilibrium constant in one sentence?
Use the focus statement above, then add one particle-level or mathematical detail.
What evidence would support an AP-style answer on this topic?
Use a balanced equation, diagram, graph, table, numerical setup, or particulate model depending on the prompt.